1. Field of the Invention
This invention relates to the field of electronic filters and amplifiers for electroacoustic systems such as hearing aids, and more particularly to methods and devices for clinical testing and for correction of hearing impairment.
2. Description of the Related Art
Hearing impairment is most commonly expressed as a loss of sensitivity to weak sounds, while intense sounds can be as loud and uncomfortable as in normal hearing. State-of-the-art hearing aids treat this phenomenon of “loudness recruitment” with sound amplification that automatically decreases with sound amplitude. This compresses the range of normally experienced sound amplitudes to the smaller range required by the impaired ear. The best engineering approach to compression has, however, been uncertain. Rapid compression amplifiers protect the ear from uncomfortable changes in loudness, but nonlinearly distort the sound waveform. Slowly adapting compression avoids the distortion, but allows some loudness discomfort.
Recent advances in hearing aid development have been largely driven by availability of inexpensive miniaturized electronic analog and digital signal processors. The classical audiological problem of loudness recruitment, which older hearing aids solved with a manual volume control, is now solved with sound compression systems that automatically provide greater amplification for weak than for intense sounds. In a recent comprehensive and authoritative review, Harvey Dillon, in Ear and Hearing 17:287-307 “Compression? Yes, but for low or high frequencies, for low or high intensities, and for what response times?” [comments by Vilchur, and reply by Dillon, 1997, in Ear and Hearing 18:169-173] found that 1) “for speech in quiet at a comfortable level, no compression system yet tested offers better intelligibility than individually selected linear amplification” (i.e., manual volume control), and 2) “In broadband noise, only one system, containing wideband compression followed by fast acting high-frequency compression, has so far been shown to provide significant intelligibility advantages.”
The need for improved hearing aids and audiological fitting procedures is widely attested to by research efforts worldwide. It has been said that over 28 million Americans have hearing impairments severe enough to cause a communications handicap. While hearing aids are the best treatment for most of these people, only about 5 million actually own hearing aids, and fewer than 2 million are sold annually. In addition, less than 60% of hearing aid owners are actually satisfied with their hearing aids.
Loudness recruitment, or loss of dynamic range, is the basic audiological problem confronting hearing aid design. Modern hearing aids automatically compress the range of sound levels into a much smaller range, as needed. Broad agreement exists that the most general and potentially successful design is a multichannel compressive hearing aid that addresses the compression needs of each band of audible frequencies. Sharp disagreement exists, however, over whether wide dynamic range compression should be instantaneous or slowly adapting.
It has been suggested that rapid compression is fundamentally flawed and that multichannel hearing aids should use a slowly acting graded volume control with approximately ¼ second attack and delay times with gradual gain reduction. This suggestion is based on the psychophysical fact that rapid compression reduces perceptually useful temporal modulation in auditory signals. It is known that loss of slow modulation (i.e., 4-16 Hz) in speech signals degrades its intelligibility. However, one study showed that the effect of rapid compression is severe only for compression ratios greater than two. Also rapid compression may be required when the residual dynamic range in the hearing impairment is smaller than the instantaneous fluctuations in normal discourse. Recent comprehensive data on speech statistics indicate that a ˜30 dB range maximum is required to include 90% of all short-term RMS samples (125 ms window), while ˜40 dB is required to capture the instantaneous speech peaks for bands of speech. Other research indicates that the latter range is relevant, so that rapid compression may be best for smaller residual dynamic ranges.
It will thus be appreciated that there is a need for more rational guidance to the design of hearing aids, and more particularly guidance that is derived from models of nonlinear cochlear signal processing. Correspondingly, there is a need for devices and methods that allow systematic audiological testing of the benefits of the new hearing aid design and fitting of individual hearing aids.